Tom Hui (l to r), acting director Department of Building Inspection and San Francisco Fire Marshal Thomas Harvey test a carbon monoxide alarm during a press conference at Star Community Homes, a project of Catholic Charities CYO on Thursday, November 29, 2012 in San Francisco, Calif.

Carbon monoxide is known as the silent killer, but on Jan. 1, millions of Californians will be safer under a law requiring landlords to provide alarms that detect the gas in apartment houses and other multi-family buildings.

About 23,000 multi-unit residences in San Francisco will be affected in this second installment of a gradual rollout of a law enacted in 2010 that requires detectors for the deadly gas, which is odorless and colorless.

Single-family and duplex homes were required to have the alarms as of July 2011.

Building and health officials hope to make the silent killer a little louder with the alarms, which beep loudly like smoke alarms. San Francisco's older buildings are particularly risky, since carbon monoxide often comes from appliances fueled by natural gas or oil.

The gas often spreads through ventilation systems in buildings. Signs of exposure begin with flu-like symptoms and get worse from there.

Alarms will be required on every floor, including basements, and should be placed in hallways outside of bedrooms. The law applies only to homes and buildings that have a gas heater or appliance, fireplace or attached garage.

The alarms do not replace smoke detectors, which are still required, though many manufacturers sell units that are both smoke and carbon monoxide detectors. Each alarm typically costs between $10 and $40. They can be found at hardware stores.

San Francisco fire crews responded to 47 cases of carbon monoxide leaks in the past year, said spokeswoman Mindy Talmadge.

Because it is not a disease that requires reporting for record keeping, it is difficult to find statistics on how many illnesses and deaths occur from carbon monoxide poisoning in the Bay Area. But about 500 people die and 20,000 are injured each year nationwide, according to the Centers for Disease Control and Prevention.

Last year, four people at the Marriott hotel at Fourth and Mission streets were hospitalized after an Indian restaurant next door began leaking carbon monoxide from a poorly repaired appliance, Shields said.Earlier this month, a hotel in Burlingame was evacuated after a man reported feeling ill and paramedics found a boiler was leaking the gas.

Neither building had carbon monoxide detectors at the time, San Francisco and Burlingame authorities said. Hotels and motels will not be required to have them until January 2016. All buildings that undergo construction or are being remodeled are required to have the alarms installed when work is completed.

Serious illness and death are relatively infrequent, but since the gas can only be detected by an alarm, it's an important life-safety issue if residences don't have working alarms, said San Francisco Fire Marshal Tom Harvey.

Landlords will be held responsible if their building doesn't have the alarms after Jan. 1, but Harvey and others say tenants should protect themselves by reminding landlords to install the alarms.

"We've been urging tenants that if the detector isn't installed, to demand it from the landlord, and if that doesn't work, to contact a local inspector to get them cited for it," said Dean Preston, executive director of Tenants Together, a California tenants advocacy group.

Poisoning

About 500 people in the United States die every year from carbon monoxide poisoning. Many occur during power outages when people try to cook or heat their homes by burning charcoal or using unvented camp stoves.

The initial symptoms of low to moderate carbon monoxide poisoning are similar to flu, but without the fever.

They include: headache, fatigue, shortness of breath, nausea and dizziness.

High level carbon monoxide poisoning results in progressively severe symptoms and can be fatal.